Field of the Invention
The present invention relates to an optical modulator and particularly to an optical modulator including a relay substrate that relays a lead pin for inputting radio frequency signals, which is provided in a package case, and an electrode of an optical modulation element and an optical transmission apparatus using the optical modulator.
Description of Related Art
In high-frequency/high-capacity optical fiber communication systems, optical modulators embedded with waveguide-type optical modulation elements are frequently used. Among these, optical modulation elements in which LiNbO3 (hereinafter, also referred to as LN) having an electro-optic effect is used for substrates cause only a small light loss and are capable of realizing broad optical modulation characteristics and are thus widely used for high-frequency/high-capacity optical fiber communication systems.
In an optical modulation element in which this LN substrate is used, Mach-Zehnder-type optical waveguides, RF electrodes for applying radio frequency signals, which are modulation signals, to the optical waveguides, and bias electrodes for performing a variety of adjustments for favorably maintaining modulation characteristics in the waveguides are provided. In addition, these electrodes provided on the optical modulation element are connected to a circuit substrate on which electronic circuits for causing modulation operations in the optical modulator are mounted via lead pins or connectors provided in a package case of the optical modulator which houses the optical modulation element.
Regarding modulation methods in optical fiber communication systems, in response to the recent trend of an increase in transmission capacity, transmission formats of multilevel modulation and transmission formats achieved by incorporating polarization multiplexing into multilevel modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) has become mainstream and been used in core optical transmission networks and also has been introduced into metro networks.
Optical modulators performing QPSK modulation (QPSK optical modulators) or optical modulators performing DP-QPSK modulation (DP-QPSK optical modulators) include a plurality of Mach-Zehnder-type optical waveguides having a nested structure, which is termed a so-called nest-type structure, and include a plurality of radio frequency signal electrodes and a plurality of bias electrodes (for example, refer to Japanese Laid-open Patent Publication No. 2016-109941), which creates a tendency of an increase in the sizes of package cases of the optical modulators. However, in recent years, conversely, a demand for the size reduction of the modulators has been intensifying.
As a measure for satisfying the above-described demand for size reduction, an optical modulator which can be electrically connected to external circuit substrates by replacing a push-on-type coaxial connector provided in the package case of an optical modulator of the related art as an interface of the RF electrode by the same lead pins as the interfaces for the bias electrode and a flexible printed circuit (FPC) which is electrically connected to these lead pins is proposed (for example, refer to Japanese Laid-open Patent Publication No. 2016-109941).
For example, in a DP-QPSK optical modulator, an optical modulation element constituted of four Mach-Zehnder-type optical waveguides respectively having RF electrodes is used. In this case, four push-on-type coaxial connectors provided in the package case of the optical modulator inevitably increase the size of the package case, but the use of the lead pins and FPCs instead of the coaxial connectors enables size reduction.
In addition, since the lead pins in the package case of the optical modulator and a circuit substrate on which electronic circuits (driving circuits) for causing modulation operations in the optical modulator are mounted are connected to each other via the FPC, it is not necessary to perform the excess length treatment of coaxial cables used in the related art, and it is possible to decrease the installation space of the optical modulator in optical transmission apparatuses.
In the above-described optical modulator including the lead pins for inputting high-frequency electrical signals in the package case, generally, the lead pins and the electrodes of the optical modulation element housed in the package case are connected to each other via a relay substrate disposed in the package case (for example, refer to PTL 1).
FIG. 8A, FIG. 8B, and FIG. 8C are views illustrating an example of the constitution of the above-described optical modulator of the related art. Here, FIG. 8A is a plan view illustrating an optical modulator 800 of the related art mounted on a circuit substrate 830, FIG. 8B is aside view of the optical modulator 800 of the related art, and FIG. 8C is a bottom view of the present optical modulator 800 of the related art. The present optical modulator 800 includes an optical modulation element 802, a package case 804 accommodating the optical modulation element 802, a flexible printed circuit (FPC) 806, an optical fiber 808 for making light incident on the optical modulation element 802, and an optical fiber 810 guiding light output from the optical modulation element 802 to the outside of the package case 804.
The optical modulation element 802 is, for example, a DP-QPSK optical modulator including four Mach-Zehnder-type optical waveguides provided on an LN substrate and four radio frequency electrodes (RF electrodes) 812a, 812b, 812c, and 812d which are respectively provided on the Mach-Zehnder-type optical waveguides and modulate light waves propagating through the optical waveguides.
The package case 804 is constituted of a case 814a and a cover 814b to which the optical modulation element 802 is fixed. Meanwhile, in order to facilitate the understanding of the constitution in the package case 804, in FIG. 8A, the cover 814b is only partially illustrated in the left side of the drawing.
The case 804a is provided with four lead pins 816a, 816b, 816c, and 816d. These lead pins 816a, 816b, 816c, and 816d are sealed with glass sealing portions 900a, 900b, 900c, and 900d (described below), extend outside from the bottom surface (the surface illustrated in FIG. 8C) of the package case 804, and are connected to through-holes a formed on the FPC 806 with solders and the like.
One end of each of the lead pins 816a, 816b, 816c, and 816d is electrically connected to each of the RF electrodes 812a, 812b, 812c, and 812d on the optical modulation element 802 via the relay substrate 818.
The other end of each of the RF electrodes 812a, 812b, 812c, and 812d is electrically terminated using a terminator 820.
FIG. 9A is a partial detail view of a C portion of the optical modulator 800 illustrated in FIG. 8A, and FIG. 9B is a cross-sectional view of the optical modulator 800 in a direction of a IXB-IXB line in FIG. 8A. The lead pins 816a, 816b, 816c, and 816d are glass terminals, extend toward the outside of the package case 804 from the inside of the package case 804 respectively through the glass sealing portions 900a, 900b, 900c, and 900d provided in the case 814a, protrude from the bottom surface (the surface illustrated in FIG. 8C) of the package case 804, and are solder-fixed to the through-holes in the FPC 806.
The lead pins 816a, 816b, 816c, and 816d are disposed in the vicinity of a side (lead pin-side edge 910) of the relay substrate 818 on the lower side of FIG. 9A (the left side of the relay substrate 818 in FIG. 9B), and are electrically connected to conductor patterns 902a, 902b, 902c, and 902d provided on the relay substrate 818 with solders 904a, 904b, 904c, and 904d respectively.
In addition, the conductor patterns 902a, 902b, 902c, and 902d are electrically connected to the RF electrodes 812a, 812b, 812c, and 812d in the lower end portion in the drawing of the optical modulation element 802 (the left end of the optical modulation element 802 in FIG. 9B), which are disposed in the vicinity of a side (modulator-side edge 912) of the relay substrate 818 on the upper side of FIG. 9A (the right side of the relay substrate 818 in FIG. 9B) by, for example, metal wires 906a, 906b, 906c, and 906d respectively.
The conductor patterns 902a, 902b, 902c, and 902d formed on the relay substrate 818 are constituted as linear patterns that are parallel to each other in order to minimize the signal propagation loss and the skew (propagation delay time difference) by minimizing the propagation distance from the respective lead pins 816a, 816b, 816c, and 816d to the respective RF electrodes 812a, 812b, 812c, and 812d corresponding to the lead pins 816a, 816b, 816c, and 816d. Therefore, the optical modulator 800 is constituted so that the intervals among the respective lead pins 816a, 816b, 816c, and 816d and the intervals among the respective RF electrodes 812a, 812b, 812c, and 812d are the same as each other.
Due to the above-described constitution, in the optical modulator 800, high-frequency electrical signals input to the lead pins 816a, 816b, 816c, and 816d from conductor patterns 832a, 832b, 832c, and 832d formed on the circuit substrate 830 via the FPC 806 are input to the RF electrodes 812a, 812b, 812c, and 812d in the modulation element 802 via the relay substrate 818.
However, in the constitution of the optical modulator of the related art, when high-frequency electrical signals propagating through the lead pins 816a, 816b, 816c, and 816d are input to the conductor patterns 902a, 902b, 902c, and 902d in the relay substrate 818 from the lead pins 816a, 816b, 816c, and 816d, the propagation direction of the high-frequency electrical signals bends, and thus, in the bent portion, some of the high-frequency electrical signals are likely to propagate in a spatial propagation mode and radiate.
That is, generally, in order to facilitate the electric connection between the lead pins and the circuit substrate 830 via the FPC 806, the lead pins 816a, 816b, 816c, and 816d are disposed so as to extend downwards perpendicular to the lower surface of the package case 804 (therefore, perpendicular to the lower surface of the case 814a). Meanwhile, the surface of the relay substrate 818 is disposed in a direction along the lower surface of the case 814a so that the relay substrate 818 is stably fixed and housed in the package case 804. Therefore, the propagation direction of high-frequency electrical signals input from the conductor patterns 832a, 832b, 832c, and 832d in the circuit substrate 830 via the FPC 840 bends so that the high-frequency electrical signals bend almost at right angles from the lead pins 816a, 816b, 816c, and 816d toward the conductor patterns 902a, 902b, 902c, and 902d in the relay substrate 818, and, in the bent portion, some of the power thereof is likely to be radiated in the space.
In addition, the radiated radio frequency signals may be coupled to the RF electrodes 812a, 812b, 812c, and 812d disposed at locations facing the bent portion with the relay substrate 818 sandwiched between the bent portion and the RF electrodes, may affect the operation of the optical modulation element 802 as radiation noise, and may adversely affect the propagation characteristics of optical signals modulated using the optical modulation element 802.